Feeler Gauges (Metrology)

2.23.
These are used to measure the width of the gap between two parallel flat faces e.g., in gauging
of the clearance between the piston and cylinder. A feeler gauge consists of a narrow strip of sheet
steel made to a given thickness. The complete set consists of a number of gauging blades of different
thicknesses assembled together. Their working depends entirely on the sense of feel. The feeler
blade should neither be forced between the surfaces nor should it slide freely, rather the correct
blade will give a characteristic ‘gauge fit’ type of feel. If necessary, two blades may be joined together
for noting any dimension.
Feeler gauges generally comprise of a series of gauging blades of different grades and
thicknesses, from 0.03 to 1 mm assembled in protective sheath. The blades are made of heat treated
bright polished tool steel having a tensile strength of about 170 kgf/mm2 for thickness up to 0.5 mm
and about 70 kfg/mm2 for thickness 0.5 mm and above. Generally the blades are available in overall
length of 100 mm. These are 12 mm wide at heel and are tapered for the outer part of their length
so that width at the tip is approximately 6 mm. These are hinged in the sheath on a screw and nut
of such a design that the blades are removable. The nut is in the form of a bush, passing through
both sides of the sheet and forms a hinge upon which the blades may be rotated. The sheath is so
designed as to fully protect the blades when not in use.
Each blade is legibly and permanently marked with the nominal thickness and grade. These
are available in two grades depending upon the accuracy. The various blades should be assembled
in such a way that each thin blade is given the maximum protection by being inter-leaved between
two thicker blades. The maximum variation in the thickness of a blade should not exceed 0.04 mm
for blades upto and including 0.3 mm thick and 0.006 mm for blades over 0.3 mm in thickness.
The recommended set by Bureau of Indian Standards which has been so devised as to furnish
sets of the greatest utility with a minimum number of blades is given below :